U.S. patent application Ser. No. 034,717 filed on Mar. 22, 1993 by the present Applicant and entitled "Ophthalmological Instrument For Producing Dichoptic Stimuli On A Visual Display Terminal" describes a method and system for presenting two successive images on a video display unit as a dichoptic stimulus of substantially equal intensity. A first image is displayed at a predetermined intensity so as to be seen by a first eye of an observer only, an optical shutter being used to prevent the displayed image from reaching the second eye of the observer. The first image is then extinguished and the state of the optical shutter reversed so that the first eye can no longer see the video display unit, whilst a second image is presented to the second eye. In order to prevent the second eye from seeing an after image of the first image, a neutral density filter is disposed within the light path, and the second image is illuminated at a correspondingly higher intensity so as to compensate for the attenuation factor of the neutral density filter.
Such a method may be employed to measure the dark vergence of the observer which has been found to be a major factor in determining fatigue of computer operators, in order that corrective action may be taken so that the angle subtended by an image on the computer screen at the eyes of the observer may be adjusted to the angle of dark vergence. Such corrective action may be provided by individually tailored prismatic spectacles.
U.S. application Ser. No. 034,717 thus addresses the specific need of computer operators and permits the dark vergence of such operators to be determined when looking in a substantially straight-ahead position towards a computer screen. Since images are presented on computer screens and the like as a raster scan, there is at any time only a single pixel which is illuminated, the effect of a complete image being obtained owing to the eye's persistence of vision. Further, since both the left and right eye images are presented on the same screen, any effect of after image must be suppressed so that each eye will see only its own image as a dichoptic stimulus.
Prior art relating to determination of dark vergence is fully described in the above-referenced U.S. application, the complete contents of which are incorporated herein by reference.
The above-referenced U.S. application and the prior art references discussed therein are, as indicated above, principally directed to the determination of dark vergence in a straight-ahead direction. This is fine for computer operators who do indeed work with their gaze directed substantially straight ahead. However, most text is, in fact, read with the eyes directed not straight ahead but inclined downwards. Fixating at text whilst looking down requires the cooperation of all twelve extraocular muscles attached to both eyes (i.e. medial rectus, lateral rectus, superior and inferior recti and superior and inferior oblique recti).
The prior art does not define the dark vergence when the eyes are not directed straight ahead in the horizontal direction. However, it may be expected that in any plane formed by the visual axis of the two eyes, a different dark vergence exists. Specifically, when looking down at an angle commensurate with reading, the dark vergence will be different to that associated with straight-ahead vision. To converge on text whilst looking down requires that some of the extraocular muscles contract and are thus under tension. This tension, when applied for several hours, can produce visual strain similar to the strain which results when an observer is looking at a visual display unit such as a computer screen.
From the above-referenced U.S. patent application three considerations are apparent: first, dark vergence varies between limits from one subject to another; secondly, dark vergence depends on the plane formed by the visual axis of the two eyes; and thirdly, the optimal distance between an operator and a visual display unit or between someone who is reading text and the text material itself depends on the subject's dark focus.
In the above-referenced U.S. patent application, a method is described in which the dichoptic stimuli to the eyes is presented so as to force each eye to see only its image. One of the images is a pair of nonious bars having a central gap and the other image is a movable dot which can be aligned by the observer with the gap in the nonious bars. When the two images are perceived as aligned by the observer, any actual displacement between the images is measured and this, together with the interpupillary distance of the observer, permits determination of the observer's dark vergence, in accordance with the following formula: ##EQU2## where: DC.sub.c is the distance of dark vergence;
P.sub.d is the interpupillary distance; PA1 D.sub.o is the distance of the optical pathway from the observer to a viewing plane of the video display unit; and PA1 .function.(T) is a polynomial function of the measured displacement T between the two dichoptic images which approximates the measured displacement T for at least a latter portion of said sequence of images. PA1 (a) generating a first one of the images at a predetermined first intensity, on the first image array so as be perceived by a first eye of the observer only, PA1 (b) generating a second one of the images at a predetermined second intensity equal the first intensity multiplied by ##EQU3## on the first image array so as be perceived by a second eye of the observer only.
Although the method and apparatus disclosed in the above-referenced U.S. application is well-suited for the determination of dark vergence of computer operators, it requires a computer which must be enclosed within an optical tunnel as shown in FIG. 5 of the U.S. application, and the resulting apparatus is therefore bulky and relatively expensive. It would clearly be desirable to extend the method disclosed in the above-referenced U.S. application so as to allow for the determination of dark vergence in directions other than straight ahead and, further, to permit such determination with a much more compact apparatus not requiring a computer. Such a simplified apparatus should clearly be within the budget of optometrists so as to allow for the determination of a patient's dark vergence as a completely standard procedure together with conventional eye tests. Then, when appropriate, special prismatic lenses may be prescribed, together with, if necessary, conventional refracting elements, in order to allow for the correction of dark vergence. Such correction is described per se in the above-referenced U.S. patent application.
It is also known that there are well known methods and systems for recording eye position or line-of-sight in the dark, some of which will now briefly be reviewed.
It is well known that there is a corneoretinal potential originating in the retinal pigment epithelium. This potential can be registered indirectly with skin electrodes placed in the horizonal plane close to the eyes' edge and, if necessary, in the vertical direction by placing one electrode above the upper eyelid and another electrode just below the lower eyelid. This method, called electro-oculography (EOG), measures the relative value of the corneofundal potential and the voltage between the electrodes which correlates with the position of the eye, separately in the horizontal and the vertical directions. Based on the measured potentials, the spatial location of the line-of-sight of each eye can be determined.
Also known is the double Purkinje eye tracker which uses the corneal reflex or first Purkinje image to determine eye position. According to this technique, a light is shined on to the eye and approximately 2.5% of the incident light is reflected from the surface of the eye to a photocell. When the eyes rotate, the reflected light changes its angle relative to the rotated eyes and the voltage generated by the photocell varies in known sinusoidal manner so that by measuring the voltage, the rotation of the eye can be determined.
Eye position can also be determined as a function of the position of the iris-sclera boundary (the limbus) with respect to the head. The ratio between the dark iris and the bright sclera observed on the left and right side of the eye may be measured directly with respective photo sensors responding to infrared light projected on to each of the eyes. This ratio is directly related to the position of the eyes in the plane formed by the two photo sensors.
Thus, in one example, respective left and right sensors are placed at eye level so as to provide a signal proportional to the eye position in the horizontal plane, whilst up and down sensors are placed above the eyebrow and below the eyelid so as to provide a signal proportional to the eye position in the vertical plane. Alternatively, eye position can be measured indirectly by projecting the image of the eye (e.g. by means of a television camera) and computing the position of the eye from this image.
It has also been proposed to image the eye with a small infrared charge-coupled device (CCD) image sensor coupled to a video tape recorder. The resulting image may be fed to a computer for analysis so as to determine the horizontal and vertical components of the eye position.
All of these methods are suitable only for determination of the eyes' orientation or position, none having any direct relevance or application to the determination of a subject's dark vergence. This having been said, the dark vergence could be determined by measuring the eyes' orientation when the subject is completely dark-adapted. However, in order to do this, it would first be necessary to dark-adapt the subject. It is not possible to employ any of the known methods for determining the eyes' orientation for determining dark vergence because, when the eyes come to rest in the dark, in the absence of any visual stimulus the eyes float so that their "line-of-sight" is essentially arbitrary. In order to employ existing techniques for measuring the eyes' orientation so as to determine therefrom a subject's dark vergence, it would first be necessary to eliminate the problem of floatation of the eyes so as to ensure that the eyes become directed along a desired line-of-sight. No prior art solution to this problem has been proposed.